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REFRACTORIES                                       489
tion of fused material is more or less pronounced. It is evident that for best resistance the refractory should be dense and as impervious as possible and not subject to chemical interaction with the liquid slag. However, it appears that not only the total volume of pore space but also its capillary character, that is, the fineness of the pores, is of great significance in this respect, as well as whether they are sealed or communicating.
The tests conducted for estimating the quality of resistance to slag corrosion consists usually in cementing a refractory ring to the surface of the test piece and placing within the space of the former a definite quantity of the slag in question. Upon heating the specimen thus prepared to a specified temperature, usually 1,350°C., the absorption of slag is noted by breaking the test piece through the center of the ring and observing the area through which the slag has penetrated. In the tentative specifications of the American Society for Testing Materials the ring is of 2.5 in. inside diameter and 0.5 in. deep. In the case of refractories for use in the glass industry small crucibles are usually made, charged with a glass mixture, more corrosive than the normal composition, and the extent of the penetration noted.
Mechanical Strength.—The mechanical strength of refractories in the cold state varies between wide limits, according to the density and hardness of the materials. Thus clay firebricks may show compressive strengths from 1,000 to 5,000 Ib. and moduli of rupture up to 1,200 Ib. per square inch. The toughness of refractories, as measured by their resistance to abrasion, likewise varies greatly. Many of the most heat-resisting refractories stand up but poorly under conditions of abrasion. Under certain conditions, as in the top part of blast furnaces, hard and tough materials are required to resist the wearing action of the charge while for other uses this requirement may be neglected. The toughness of refractory bricks may be determined by means of the "rattler/7 a machine employed for the testing of paving bricks, with the modification that the entire charge consist of the smaller spheres employed in the standard test of the American Society for Testing Materials.
Electrical Resistance and Dielectric Strength.—These qualities are involved only where refractories are used also as insulators as in electric furnaces, stoves, hot plates, heaters of all kinds, spark plugs, insulators used in Cottrell installations for the removal of dust from heated gases, etc. It is necessary that products so used have as high an electrical resistance as possible at the temperature reached and also good dielectric strength where high voltages are employed and there is danger of puncture. Refractories high in fluxes such as feldspar are apt to break down at comparatively low temperatures. In the case of porcelain it was found by Henderson and Weimer that at 275°C. a body fired to cone 9 showed a reduction in the puncturing voltage to one-thirtieth of the initial value at atmospheric temperature.1
For the testing of refractories with reference to their electrical conductivity at higher temperatures a cup of 60 mm. outside diameter, 65 mm. height and having a wall 2.5 mm. thick is made from the material in question and fired. This specimen is placed in a suitable furnace. Electrical contact is provided by the use of molten metal on the inside of the cup and its immersion in a shallow metal bath on the outside. An alternating current, 60 cycle, of 500 volts is employed and the current passing through the material measured by means of a sensitive milliammeter or a dynamometer watt meter. The temperature is determined by means of insulated thermocouples immersed in the molten metal on the inside of the cup.
i Trans. Am. Ceram. Soc., Vol. 13. p. 469-75.